Consumable items in printing devices mostly include “marking agent” components that are consumed with each printed page as part of the printed product, and rotating components that deteriorate over time as a result of wear and tear. Marking agent consumables include ink, wax, powder toner, thermal agents, and the like. Marking agents are often housed in some type of cartridge, such as a toner cartridge, and dispensed onto rotating components that transfer the agents to a print medium during a printing process. As a marking agent is depleted, it is useful to have information about the amount of agent remaining in a cartridge in order to approximate the number of pages available for printing during the remaining life of the cartridge. Various methods exist that provide information regarding the remaining useful life of a cartridge.
A page counting method does not involve direct measurement information about the level of toner (i.e., marking agent) present in a cartridge. Instead, this method provides an expected life span for a toner cartridge measured by the number of pages that the cartridge is expected to print. The life span is reduced by one page for each page that is printed. A disadvantage of this simple method is that it can be inaccurate.
The inaccuracy of this method can result from at least two factors. First, the expected number of pages available from a toner cartridge is a rough estimate set by the cartridge designer based on numerous examples of like cartridges. The actual number of available pages can vary significantly from cartridge to cartridge. Second, the amount of toner put on each printed page may vary dramatically from page to page. These factors often mean that more or less toner is left in a cartridge than expected, which can result in significant differences in the number of pages expected to be printed and the number of pages that can actually be printed by a given cartridge.
A pixel counting method also does not involve direct measurement information about the level of toner present in a cartridge. Rather, this method starts with an assumed maximum number of pixels available to be expended over the life of the cartridge. In a color laser printer, the number of pixels expended can be estimated by tracking the number of laser pulses used to magnetize a photoconductor drum. Four laser pulses will magnetize the drum to statically charge one pixel. The number of laser pulses can be measured for each printed page, and the appropriate number of pixels can be subtracted from the maximum pixels available, thereby providing a measure of the percentage of pixels (i.e., toner) remaining in the cartridge.
Unfortunately, this method suffers disadvantages similar to the previous method. The maximum number of available pixels is assumed by the cartridge designer based on numerous examples of like cartridges. The actual number of available pixels can vary significantly from cartridge to cartridge. In addition, counting laser pulses can be an inaccurate method of determining the number of expended pixels on a printed page. Although the error per page may be slight, it can add up over the life of the cartridge (e.g., 10,000 printed pages) and create a significant difference in the expected percentage of toner available and the actual percentage of toner available. Moreover, the largest errors are seen toward the end of the cartridge life cycle, which is the time when it is most important to have accurate toner level information.
Another method of determining the toner level within a cartridge utilizes antenna sensor technology. This method provides direct information about the level of toner in a cartridge by passing current from one end of the cartridge to the other through an antenna. The current induces voltage signals in coils within the cartridge that are proportional to the amount of toner present in the cartridge. Although this direct measurement of the toner level is beneficial, it too has disadvantages.
Most toner level antenna sensors provide readings that have a wide degree of deviation. The jostling or movement of toner within a cartridge can cause the voltage signals from the antenna sensor to vary by as much as 16 to 20 percent between readings. Toner movement is significant in carousel-arranged cartridges that rotate during a color print process to provide access to the different color toners. The deviation in toner readings can occur as often as every sample interval, such as after every printed page. Reported toner levels may jump drastically and provide no real sense of the actual toner level. For example, a user may notice that the reported toner level jumps from a lower value to a higher value even though significant printing has occurred between the two readings. Therefore, for any given toner level reading, a user is not really sure of how much toner remains in the cartridge. In addition, such deviation does not provide a uniform decrease in the reported toner level over time, and therefore does not ensure a level of predictability as to the remaining life of a toner cartridge.
Accordingly, the need exists for a way to accurately determine the amount of toner within a toner cartridge and to report this information in a manner that permits predictability as to the remaining useful life of the toner cartridge.